The present invention relates to a cordless telephone.
In the ETSI Committee in Europe, specifications called CT-2 are under investigation as standards for digital cordless telephone.
In the CT-2 system, there are provided 40 channels in an 800 MHz band. For example, while talking over the telephone, a parent telephone and a child telephone are connected by a channel and the channel is used for transmission and reception by being time shared in the transmission frame T and the reception frame R as shown in FIG. 7. All the prepared 40 channels are talking channels with control data transmitted and received as attached data to the speech data. Hence, there exists no independent control channel.
In the CT-2 system, telephones can be used versatilely than with ordinary cordless telephones by having them previously registered. More specifically, for example as shown in FIG. 8, if a plurality of child telephones 11 to 1j are previously registered with a parent telephone 21, the parent telephone 21 can manage both an outgoing call and an incoming call through a line wire for any of the child telephones 11 to 1j. Further, if a child telephone 11 is previously registered with a plurality of parent telephones 21 to 2i, the child telephone 11 can send a call to and receive a call from a line wire through any of the parent telephones 21 to 2i.
The cordless telephone on the CT-2 system individually has a narrower service area than that of the cordless telephone on the general cellular system but is simpler in system structure and economical in terms of the telephone charges. Hence, if it is used within an office block or the like, it can provide means for simple communications despite up the narrowness in service area.
When an outside call comes in to the above described CT-2 system, it is unknown to the child telephone which parent telephone (of those, with which the child telephone is registered) will request the child telephone to receive the call and which channel the parent telephone will use. Therefore, the child telephone on standby must repeat sequential receiving of all of the 40 channels to get ready for a request from a parent telephone for receiving an incoming call.
Then, however, the battery power supply to the child telephone is greatly consumed. Hence, the usable period of time of a once charged battery becomes considerably short.
Therefore, a method which alternately performs a scan mode and a sleep mode is considered. The scan mode scans all of the channels sequentially only once. The sleep mode stops, for a fixed period of time, the operations of all of the circuits except the system controller. By using this method, the battery can be suppressed from being consumed because even the system controller consumes little power during the period in the sleep mode.
In such case, however, any appreciable effect cannot be obtained unless the period in the sleep mode is set sufficiently long as compared with the period in the scan mode. Further, since as many as 40 channels are sequentially received in the scan mode, the time required for the scan mode becomes relatively long.
Thus, in order to effectively suppress the power consumption of the battery, the cycle of the scan mode plus the sleep mode becomes considerably long. Then, when a request for receiving an incoming call is transmitted from a parent telephone, a long time elapses before the child telephone accepts the request for receiving a call, and therefore this method has little practicability.
Furthermore, since there are as many as 40 usable channels for talking, the degree of freedom in selecting a channel becomes high for both the child telephone and the parent telephone. Accordingly, it is difficult, conversely, to select one channel out of them. Hence, a long time is taken, for example, in receiving an incoming call as described above.